This invention relates to a process for the crystallization of melts of chemical products in double-shaft, coolable worm machines whose worm shafts rotate in the same direction. The invention also relates to an embodiment of a worm machine which is suitable for carrying out the process.
Crystallizing worms are used successfully, for example, in the chemical industry, when hot, for example heavily fuming or harmful, organic melts are to be converted into the cooled, solid phase in a completely closed apparatus without any contact with the atmosphere. Crystallizing worms are also effective in the case of difficult crystallizing melts which tend to supercool, because in this case, solidification does not take place in complete rest but with continuous movement and shearing and thus the solidification process may be carried out more easily and in an accelerated manner.
Worm machines for crystallizing functions of this type are known. For this purpose, self-cleaning, tightly threaded double worms are used, whose two worm shafts rotate in the same direction and which have a cooled housing. They are described in detail in "Schneckenmaschinen in der Verfahrenstechnik" (Springer-Verlag, 1972) by H. Herrman, [1].
A more recent special development of such worm machines is the double-shaft worm heat exchanger whose shafts rotate in the same direction, that is the ZDS-W produced by Werner and Pfleiderer, which was disclosed in a company brochure [2] 1979. In addition to the housing being cooled, this apparatus offers cooling surfaces for crystallizing processes due to additionally intensively coolable hollow worm shafts and which are continuously scraped off and which are very large. The kinematic coercive self-cleaning operation of all the cooling surfaces prevents the chamber which may be filled with the product from becoming completely blocked against transport of the material by solidification of the melt. There is a continuous force transporting effect in the threaded worm shafts.
Worm machines require for the material transport and the internal shearing procedures a drive power which, in the case of double worms, must also be provided for scraping off the surfaces of the worms. This drive power is converted into frictional heat and it causes the material to heat up. During the crystallization process in worm machines, in addition to the crystallizing heat, this frictional heat also has to be removed over the cooling surface which causes an additional strain on the energy balance of the apparatus. Therefore, attempts have been made to minimize the frictional heat which is itself harmful by a worm geometry which is low in shearing action. In the case of the crystallizing worms mentioned above under number [2], this is effected by a steadily continuous worm thread profile which is low in shearing action (a short counterthread for protecting the shaft sealing against product intrusion is only provided in the open outlet shaft for the solidified crystallized material at the end of the machine). Intensively working mixing and shearing elements known in worm technology according to [1], for example kneading elements, are understandably not used in this case.
However, it has been shown in the practical chemical operation of the crystallizing worms which have been described, that as a result of material movements in the machine which are unusual per se and are particular to this case, the efficiency thereof is greatly restricted. Where there are economic passage conditions, i.e. from medium to high throughputs, large C-shaped product particles which have not completely solidified all through (so-called "small horns") are formed, the largest of which correspond to a negative shape of the worm channel (G. Matz, Chem. Ing. Tech. 52, (1980), P. 570-575). These particles have completely solidifed on their contact surface with the cooling surfaces of the worm and thus they form a stable crust. However, they still contain, enclosed inside, liquid melt or plastic crystal sludge. The particles are so stable because of their surrounding crust that they withstand the friction and pressure forces which are present in the worm and they are transported too rapidly through the machine, because of their C-shaped negative worm channel form, with the maximum possible conveyance of a worm, namely with the so-called screw-nut conveyance. The C-shaped particles mentioned are ejected from the end of the machine and they reheat considerably due to the recrystallization of their plastic core and thus they exhibit a considerable cementing tendency in a stationery discharge. These phenomena are particularly pronounced in the case of rapidly crystallizing materials. A slightly different conveying mechanism produces round particles which, with a maximum diameter equal to the depth of the worm thread, rotate in the worm channel like the balls in a rolling bearing, which effect may also take place on figure-of-eight-shaped tracks in the case of double worms rotating in the same direction. In this case as well, the same effect of a melt inclusion occurs with the above-described negative consequences. Only where there are very low and thus uneconomic throughputs may the required temperatures and properties of the bulk material which is discharged be achieved in the cases which have been described.